1. Field of the Invention
The present invention relates to an image processor unit, an image processing method, and a storage medium.
2. Description of the Related Art
Conventionally, studies have widely been made regarding the technique of multiplexing image-related additional information into video information. Recently, standardization is in progress for a technique called an “electronic translucent technique”. According to the electronic translucent technique, video information, such as a photograph and a picture, is added by additional information. The additional information includes, for example, the name of the originator thereof and use-permission therefor. The additional information is multiplexed into the image so as not to be easily identified by the human eye. The image thus created is distributed via a network such as the Internet.
As one of other application fields, in advances in image outputting machines, such as photocopy reproducing machines and printers that are capable of outputting high quality images, techniques are developed to prevent illegal counterfeiting of, for example, paper money and revenue stamps. According to one of these techniques, additional information is embedded in an image so that an output machine and a serial number thereof can be identified from an image printed on paper. For example, additional information is embedded in a high frequency band for a low-visual-sensitive color difference component and color saturation, and the information is thereby multiplexed into an image.
However, the above-described techniques are problematic as described below. FIG. 19 illustrates embedding of general additional information according to an electronic translucent technique. Video information A and additional information B are multiplexed through an adder 1901, and are thereby changed to multiplexed information C. The illustration in FIG. 19 represents an example case in which additional information is multiplexed into a real space zone of video information. If the multiplexed information C can be distributed without performing image processing such as various filtering, and encoding such as nonreciprocal compression, the additional information B can easily be decoded from the multiplexed information C according to the conventional techniques. For video information distributed through the Internet, if it has some noise resistance, the decoding thereof can be performed through a digital filter capable of improving image quality in edge enhancement, smoothness, and the like.
However, a case is assumed in which a multiplexed image is printed by an output machine such as a printer, and additional information is retrieved from a printed object that has been output therefrom. In addition, the printer used in this case is assumed to have only the capacity of representing two or several tones in units of a single color.
In recent years, inkjet printers capable of representing several tones in color units are marketed. For example, one of the inkjet printers includes low-dye-density ink, controls the diameters of output dots so as to be variable, and thereby represents the tones. However, unless pseudo-gray-scale processing is performed, photograph-level image tones cannot be represented. Specifically, in the assumed case where the additional information multiplexed using the electronic translucent technique shown in FIG. 19 is output to the printer, as shown in FIG. 20, the multiplexed information C is changed to quantization information D according to a pseudo-gray-scale processing 2001. Subsequently, the quantization information D is printed by printer output 2002 on paper. Thereby, the input information is changed to on-paper information E (printed object) significantly degraded in quality.
Accordingly, the decoding of the additional information from the on-paper information to prevent the aforementioned counterfeiting is equivalent to the decoding of the additional information B from the on-paper information E subjected to series of the processing shown in FIG. 20. The amount of variations caused during the pseudo-gray-scale processing 2001 and the printer output 2002 are significantly great. The significantly grate variations make it difficult to perform multiplexing so that the additional information cannot be visually identified, the multiplexed additional information from the on-paper information.
FIG. 21 shows an example conventional electronic translucent technique in which video information is transformed into a frequency band instead of the real space zone by using, for example, a Fourier transformation method, and is synthesized with a high-frequency band. In the figure, video information is transformed by an orthogonal transformation processing 2101 into a frequency band, and additional information is added by an adder 2102 to a specific frequency that cannot be easily visually perceived. After the information is returned by reverse orthogonal transformation processing 2103 to a real space zone, it is passed through filters similar to those shown in FIG. 20, such as pseudo-gray-scale processing and printer output, which involves great variations.
FIG. 22 shows a procedure for separating additional information from paper. In the procedure, the information contained on the printed object is input through an image scanning unit, such as a scanner. Since the input information is represented in gradation according to the pseudo-gray-scale processing, it is subjected to reverse pseudo-gray-scale processing, that is, reproduction processing 2202. Generally, the reproduction processing 2202 uses a low pass filter (LPF). The decoded information is subjected to orthogonal transformation processing 2203. Then, separation processing 2204 is performed to separate at the additional information from a power having a specific frequency.
As is apparent from FIGS. 21 and 22, many complicated steps are performed to multiplex the additional information and to separate it. For a color image, the procedure additionally includes color conversion processing for converting colors to printer-specific colors. To implement favorable separation in the complicated procedure, a signal having a very high resistance needs to be input. However, to maintain high image quality, the input of highly-resistible signal is not suitable. In addition, the procedure including many complicated steps takes a long processing time for multiplexing the information and separating it.
In addition, the decoding tends to fail in the above-described technique. According to the technique, the information is added to a high frequency band through the information-multiplexing processing performed through the embedding of the additional information in the high frequency band of a low-visual-sensitive color difference component and color saturation. In the technique, however, in a case where an error diffusion method is executed in the latter pseudo-gray-scale processing, the band of the additional information is buried in the band of a texture developed during error-diffusion. This causes the decoding to fail. Furthermore, the decoding in the above-described technique requires a scanner unit capable of producing significantly high precision. Therefore, the method shown in FIGS. 20 and 21 is not suitable in the case where the pseudo-gray-scale processing needs to be performed as a prerequisite condition. In other words, the aforementioned case requires an additional-information multiplexing method that greatly relies on the characteristics of the pseudo-gray-scale processing.
Two techniques are described below by way of examples in which additional-information multiplexing and redundancy in pseudo-gray-scale processing are correlated.
According to the first technique, when an organizational dither method is performed for binary conversion, one of dither matrixes representing identical gradations is selected, and data is included in a video signal. However, according to the organizational dither method, without a printer capable of printing high resolution data and having high mechanical precision, it is difficult to output a photograph-level high quality image. This is because even a slight deviation in mechanical precision causes low frequency noises, such as lateral stripes that are easily developed to become visually identifiable on paper.
In addition, the dither matrix is periodically varied, the band of a specific frequency generated according to regularly arranged dithers is disordered, and the image quality is adversely affected thereby. The gradation representation capability significantly depends on the type of the dither matrix. Particularly, on paper, since variations in area ratio in dot-overlapped portions are different depending on the dither matrix, even in a region having a uniform density in terms of a signal, the density can be varied when the dither matrix is changed.
Furthermore, according to the decoding method wherein a dither matrix responsible for binary conversion is inferred in a state where pixel values of video information corresponding to an original signal is unknown, a decoding side (separating side) tends to perform erroneous decoding at a significantly high probability.
The second technique uses a color dither pattern method that performs the multiplexing of additional information into an image according to the arrangement of dither patterns. Also in this method, similarly to the first technique, degradation of image quality cannot be avoided. Compared to the first technique, in the second technique, while a greater amount of additional information can be multiplexed into the image, the color vision is varied according to variations in the arrangement of color components. Particularly, this increases the level of image-quality degradation in a flat portion. In addition, this method can further increase the difficulty in the decoding of on-paper information.
Either of the methods in which the dither matrix is changed is problematic in that, the image quality is significantly degraded, and in addition, the decoding is difficult.
In view of the above, the applicant of the present invention proposed a method in which combinations of quantization values that cannot be generated in ordinary pseudo-gray-scale processing is artificially created by using textures generated according to an error diffusion method, and symbols are thereby embedded.
According to this method, although microscopic variations slightly occur in the shape of the texture, the visual image quality is not deteriorated. In addition, when a method for changing a quantization threshold in an error diffusion method is used, since visual effects of area-degradation density values are maintained, multiplexing of a different-type signal can be implemented very easily.
However, according to the above-described proposal, a decoding side needs to determine as to whether or not the texture is artificial. For a printed object output onto paper, because of positional deviations, such as biases, from desired landing positions of dots, a case can occur in which the texture cannot be reproduced in a good condition. For a color image, the identification of a texture in a real space zone can easily be influenced by other color components, thereby making it difficult to separate multiplexed information.
For example, in a method developed to embed a significantly large amount of information, such as audio information, in an image, the audio information is converted into a dot code, which is a so-called two-dimensional barcode, and the dot code is printed in a marginal portion of the image or an interior portion of the image. However, in this method, the dot code and image information are not multiplexed as two different types of information, and the code is not processed so that it cannot be easily visually identified. As only example method for making it difficult to visually identify a code, a method in which a transparent paint is used to embed the code in an image has been proposed to date. However, since special ink is required, not only costs are increased, but also the quality of an image printed on paper is degraded as a matter of course.